In the domain of civil aviation, landing assistance systems exist allowing an aircraft to fly precision approach operations. These systems provide an indisputable operational benefit by guiding the aircraft in reliable manner, thanks in particular to vertical guidance up to a decision height corresponding with minimum heights, in general less than or equal to 200 feet (approximately 60 meters), in function of the category (cat-I to cat-III) of the intended precision approach.
These minimum heights are even zero for category Cat-IIIC approaches. Some of said approaches can end in an entirely automatic landing. The main existing landing assistance systems, used for executing precision approaches, are ILS (“Instrument landing System” in English) and MLS (“Microwave Landing System” in English). These landing assistance systems are relying on one or more ground stations dedicated to this function and on means, aboard the aircraft, for receiving the signals emitted by said ground stations.
Landing assistance systems also exist using aircraft position information determined by means of a GNSS satellite positioning system, in which said position is compared with a reference path corresponding with the anticipated approach. The precision, the integrity, the continuity and the availability of the position information used by said landing assistance systems can be improved by so-called augmentation techniques. These techniques were defined in particular by the OACI (“International Civil Aviation Organization”). It is for instance possible to use GBAS type augmentation (“Ground based Augmentation System” in English), as in the GLS landing assistance system (“Ground based augmentation landing System” in English), in order to carry out precision approaches.
One future objective is to be able to execute precision approaches, which could include automatic landing, in any type of airport, therefore also in airports not equipped with ground stations, such as for instance the stations used for ILS or MLS systems. For this purpose, it will be necessary to use landing assistance systems using aircraft position information determined starting from a GNSS type satellite positioning system, and augmentation techniques which do not necessarily rely on ground stations inside the airport. These augmentation techniques can be, for instance, SBAS type techniques (“Satellite Based Augmentation System” in English) or ABAS type techniques (“Airborne Based Augmentation System” in English) defined by the OACI. This last type of augmentation relies in particular on RAIM type techniques (“Receiver Autonomous Integrity Monitoring” in English) and/or AAIM type techniques (“Aircraft Autonomous Integrity Monitoring” in English). SBAS type augmentations can be implemented by means of systems such as WAAS (“Wide Area Augmentation System” in English) in the USA, or EGNOS (“European Geostationary Navigation Overlay System” in English) in Europe.
In the approaches relying on landing assistance systems using aircraft position information determined starting from a GNSS type satellite positioning system, said position information is calculated in 3 dimensions starting from distance measurements, called pseudo distance between GNSS satellites and one or more GNSS receivers on board of the aircraft. The performance and the behavior in time of the GNSS position in 3 dimensions depend on different error contributors linked to the satellite constellation, the propagation effects of the GPS signal through the atmosphere and the quality of the receiver on the one hand, and on the GNSS constellation geometry on the other hand. For instance, in the case of the existing GPS constellation, a user receiver sees satellites rising and/or setting as result of their orbit around the earth, whereby the orbit consists of one revolution in 23 hours and 56 minutes. A satellite can also be removed from the calculation of the GPS based position because of a defect of said satellite detected by the receiver thanks to an augmentation system such as SBAS, GBAS or ABAS. This addition or removal of one or more satellites in the calculation of the aircraft position, during the approach, can create position jumps of a few meters. These position jumps, which are acceptable in the context of approaches limited to a decision height, for instance of 200 feet (approximately 60 meters), might not be acceptable for precision approaches with lower decision height, in particular in case of automatic landing. In fact, given the required high performance for guiding an automatic landing, it is necessary to know with great precision (a few meters) the position of the aircraft and a position jump could be assimilated with a strong bias, which in some cases might not be acceptable.